Electric vehicles (e.g., hybrids, plug-in hybrids, and fully electric vehicles) are one of the transportation sector's solutions to meeting future increased fuel economy standards. Electric vehicles often depend on batteries, such as lithium-ion (Li-ion) and nickel metal hydride (NiMH) batteries, to displace some or all of the vehicle's traction or auxiliary electrical loads. During use, sealed (e.g., Li-ion) or re-sealable (e.g., NiMH) batteries may experience gassing as an unintended by-product of either proper or improper operation. Over time, this gassing can generate significant internal pressure in the individual cells of a battery.
This internal pressure can lead to a detrimental cycle of degradation as the pressure change will promote electrode damage. The damage may be particularly harmful in pouch-style cells. In the case of pouch or rigid case cells (e.g., prismatic or cylindrical cans), the pressure may reach such a point that a one-time use pressure release port is opened. Once the pressure release port is activated, the cell is generally considered no longer useable, since the outside environment has likely contaminated the cell internals. Other than single-use pressure release ports, the method of mitigating gassing that is generally used is to put the cells through several formation cycles prior to sealing, thereby allowing any gas generated during these cycles to escape.